Orthotics are prescribed for a broad range of muscular or joint-related inefficiencies and ailments. An orthotic device may be custom made or made in standard sizes. The customized fit is usually achieved by using a positive mold of the portion of the body over which the orthotic device is intended to be worn. One current practice is to cast the body part with plaster of paris. Once removed from the body, the cast is called a negative model. The cast is dammed off and filled with a slurry mixture of plaster of paris and is allowed to harden. Then, the cast is removed and a plaster edition of the body part is provided called a positive model.
Three general types of materials can be used to shape the orthotic device once a positive mold of the body segment has been provided: thermoplastic resin materials, thermoplastic composite materials, and thermoset composite materials. Each type of material has its own inherent advantages and difficulties.
A primary advantage of thermoplastic resin materials is ease of construction of the orthotic or prosthetic device. These materials can be vacuum thermoformed on the positive mold to thereby provide a customized device. One current fabrication method is to use a monolithic sheet resin, such as polypropylene, heated in a convection oven and drape molded over the positive model. An internal vacuum source assures compliance of the thermoplastic to the positive model. Vacuum thermoforming provides an intimate fit and the transmission of very fine corrective forces from the orthosis to the body segment.
One drawback with thermoplastic resin materials stems from the material is flexibility which makes vacuum thermoforming possible. Thermoplastic resin materials tend to suffer from a lack of ultimate strength and rigidity. To ensure proper rigidity, orthotic devices constructed entirely from thermoplastic resin materials must be quite thick. If too thin, the device may be of limited corrective benefit. For example, when polypropylene is used in a lower extremity device such as an ankle foot orthosis (AFO), the weight of the patient may be sufficient to overcome the rigidity of the orthosis. The corrective benefit inherent in the design of the orthosis would be at least partially lost.
Unlike thermoplastic resin materials, thermoset composite materials can be very rigid and strong. In these materials, layers of fibrous materials impregnated with a thermoset adhesive are used to construct the orthotic or prosthetic device. As disclosed in U.S. Pat. No. 4,439,934, the strength and rigidity of the device can be customized by varying the types of fibers used, and by varying the nap of the fiber layers relative to the stress lines in the orthotic device.
Thermoset composite materials, however, suffer from extremely cumbersome and labor intensive fabrication techniques. Typically, each fiber layer must be individually sized and draped on the positive mold. The final composite is then cured under appropriate conditions.
Thermoset composite materials suffer from two additional difficulties. First, the fiber materials used in constructing the composites can make the final orthotic device very heavy, especially in comparison to devices made solely from thermoplastic resin materials. Second, the fiber materials used to construct the laminated composite are themselves expensive, and greatly increase the cost of the final orthotic device.
As disclosed in U.S. Pat. No. 4,778,717, some thermoplastic composite materials have been used to construct orthotic devices. In a typical construction, a core of thermoplastic material is encapsulated within layers of fiber materials impregnated with the same thermoplastic.
Orthotic devices constructed from these thermoplastic composite materials may avoid some of the problems associated with pure thermoplastics and with thermoset composites. For example, unlike thermoset composites, the thermoplastic composite can be vacuum thermoformed, thus permitting the ease of construction associated with pure thermoplastic materials. The use of external fiber layers increases the strength and rigidity of the orthotic device, thus overcoming the inherent lack of rigidity and strength associated with pure thermoplastics.
Although some of the problems associated with pure or unreinforced thermoplastics and with thermoset composites are overcome by using thermoplastic composite materials to fabricate orthotic devices, these materials nonetheless present some difficulties. The fibers used in the thermoplastic composite make the entire orthotic devices strong and rigid. Thus strength may be provided where it is not particularly needed thereby adding unnecessarily to the weight and material cost of the orthotic device.
Prosthetic devices are manufactured devices for replacing a missing body part, such as a limb or portion thereof. Prosthetic devices that are composed of endoskeletal components typically use a thermoplastic socket for the interface with the residual limb. Endoskeletal prosthetics have components between the socket and the prosthetic foot typically made of metal or plastic materials covered with a non-load bearing soft foam cover shaped to an anatomical profile. The cover is more lifelike than exoskeletal prosthetic devices. Thermoplastic sockets are susceptible to the same disadvantages as orthotic devices when the sockets are made of thermoplastic sheet material in a thermo vacuum-forming process or of other materials as previously described.
An improved material and method is needed to overcome the problems associated with the materials currently used in fabricating orthotic and prosthetic devices. Specifically, it would be advantageous to provide a material and method which can provide an orthotic device that has sufficient strength and rigidity in needed areas without unnecessarily adding to the weight and material cost of the orthotic device.